Abstract
The H I in the outer Galaxy is reanalyzed using the CO rotation curve and the full latitude extent of the gas from the Weaver and Williams survey. Gas with a surface density > or approx. =0.1 M/sub sun/ pc/sup -2/ is found to a distance of 30 kpc from the center. Three distinct, well-defined spiral features of roughly constant surface density are seen, two of which extend at least 20--25 kpc along their length and can be traced to 20 kpc from the center. If they are logarithmic spirals, the major arms have a pitch angle of approximately 22/sup 0/--25/sup 0/. A nearly circular radial corrugation is seen as a derivation from the large-scale warping at Rroughly-equal11 kpc. The outermost parts of the Galaxy show a remarkable scalloping with a large azimuthal wave number (mroughly-equal10). The scale height of the gas shows an almost linear increase from the solar vicinity to Rroughly-equal30 kpc. This increase implies that the large mass in the outer Galaxy implied by the rotation curve does not reside in the disk. The vertical distribution of the gas is shown to be not well descibed by either a Gaussian or an exponential.
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